A switching power supply including a switching element is used in a wide variety of applications as a direct-current or alternating-current power supply. As an example, the switching power supply is used as a power supply for lighting. Specifically, in recent years, in a luminaire, more and more illumination light sources are changed from an incandescent lamp and a fluorescent lamp to energy-saving and long-life light sources such as a light-emitting diode (LED). For example, new illumination light sources such as an electro-luminescence (EL) and an organic light-emitting diode (OLED) have been developed. The light output of these illumination light sources depends on a value of a flowing electric current. Therefore, when lighting is lit, a power supply circuit that supplies a constant current is necessary. Further, in order to adjust an input power supply voltage to a rated voltage of an illumination light source such as an LED, it is also necessary to convert the voltage. As a highly-efficient light source suitable for power saving and a reduction in size, a switching power supply such as a DC-DC converter is known. A discharge lighting circuit of a current push-pull type that can suppress a ripple component of an input current and obtain a high rising voltage ratio is proposed (see, for example, JP-A-08-045680).
However, in a DC-DC converter of the current push-pull type, it is necessary to control a pair of switching elements not to be simultaneously turned off. If the switching elements are caused to oscillate in a self-excited manner, complicated control is necessary.